The present invention relates to an exposure technology and, more particularly, to a technology which is effective when applied to a transfer technology for a circuit pattern in a manufacture process for manufacturing a semiconductor integrated circuit or the like.
As the high integration of a semiconductor integrated circuits advances so that the design rules of circuit elements and wiring lines reach the order of sub-microns, the photolithography step of transferring a circuit pattern on a mask to a semiconductor wafer by making use of a light of g- or i-line is accompanied by a serious problem in a reduction of the accuracy of the circuit pattern to be transferred to a wafer. In case, for example, a circuit pattern having transmissive regions P1 and P2 and shielding regions N and 252 formed on a mask 251, as shown in FIG. 13(a), is to be transferred to the wafer, lights which have just been transmitted through the paired transmissive regions P1 and P2 across the shielding region N are in phase, as shown at (b) in the same Figure, so that the two lights interfere with each other at the intrinsic shielding region on the wafer. This serves to intensify each other (as shown at (c) in the same Figure). As a result, the projected image on the wafer has its contrast degraded and its focal depth becomes shallower, as shown at (d) in the same Figure, so that the pattern transfer accuracy is seriously deteriorated.
As a means for solving this problem, there has been proposed a phase shift technology for preventing the drop of the contrast of the projected image by changing the phase of the light to transmit through the mask. In Japanese Patent Laid-Open No. 173744/1983, for example, there is disclosed the phase shift technology which will be referred to as the "Levenson type" or the "complementary phase shift method"), in which one of a pair of transmissive regions across a shielding region is formed with a transparent film so that a phase difference may be established at the time of exposure between the lights which have been transmitted through the two transmissive regions to weaken the interference light at the portion intrinsically forming a shielding region on the wafer. Specifically, one of the paired transparent regions P1 and P2 across the shielding region N (other than the shielding film 254) is formed with a transparent film 255 having a predetermined refractive index, when a circuit pattern formed on a mask 253, as shown in FIG. 14(a), is to be transferred to the wafer. By adjusting the thickness of that transparent film 225, moreover, the lights which have been individually transmitted through the transmissive regions P1 and P2 establish a phase difference of 180 degrees, as shown at (b) in the same Figure, so that they interfere with each other to weak themselves in the shielding region N on the wafer (as shown at (c) in the same Figure). As a result, as shown at (d) in the same Figure, the projected image on the wafer can have its contrast, resolution and focal depth improved to improve the transfer accuracy of the circuit pattern formed on the mask 253.
In Japanese Patent Laid-Open No. 67514/1987, on the other hand, there is disclosed a phase shift technology (which will be referred to as the "sub-shifter type phase shift method"), in which the amplitude distribution of a light having transmitted through a transmissive region of a mask is prevented from extending transversely by removing a portion of the shielding region to form fine opening patterns, by forming a transparent film in one of the opening pattern or the transmissive region existing in the vicinity of the former, and by establishing a phase difference between the light having transmitted through the transmissive region and the light which has been transmitted through the opening pattern.
In Japanese Patent Laid-Open No. 140743/1990, there is disclosed a phase shift technology (which will be referred to as the "edge emphasized type phase shift method"), in which a phase shift boundary portion is emphasized by forming a phase shifter in a portion of the transmissive region of a mask to establish a phase difference in the transmission light.
In Japanese Patent Laid-Open No. 247647/1990, moreover, there is disclosed a technology in which a g- or i-line resist is used as a phase shifter film.